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11Composition, classification, andproperties of petroleumAnalyzing petroleum samples collected from around the world would show that theirelemental compositions vary over only a narrow range: 82–87% carbon, 11–15%hydrogen, with the balance being oxygen, nitrogen, and sulfur. Oxygen and nitrogenseldom exceed 1.5% each, but sulfur can amount to as much as 6% in extreme cases.Yet these samples could show remarkable diversity in physical characteristics, rangingfrom lightly colored, free-flowing liquid to dark, smelly, highly viscous material.A consequence of the wide variability of physical properties coupled with a very narrowrange of composition is that carbon content cannot be used as a simple predictor ofproperties; in this respect, petroleum is very different from coal (Chapter 17). If thesesame samples were analyzed to determine the specific chemical compounds present ineach, any particular sample would be found to contain some 105individual components,varying in concentration from one sample to another. That is, on an elemental basis,most oils have about the same composition, but on a molecular basis no two are exactlyalike. These seemingly disparate properties arise because most components of petroleumbelong to a small number of homologous series of compounds, of which the composition,on a weight percent basis, varies only little, even over a long span of the series. Forexample, pentane, C5H12, is 83.3% C and 16.7% H on a weight basis; pentadecane,C15H32, is 84.9% C and 15.1% H. All possible isomers of all the alkanes between pentaneand pentadecane amount to 7666 compounds, yet their elemental compositions changeby only 1.6 percentage units. The range of boiling temperatures of the components spansat least 550±C; pentane, the smallest alkane liquid at room temperature, boils at 36±C,and some oils contain material that does not boil even at 600±C.11.1CompositionPetroleum contains four classes of compound: alkanes, cycloalkanes, aromatics, andheteroatomic compounds with one or more atoms of nitrogen, sulfur, and/or oxygen.In petroleum chemistry and technology, alkanes are called paraffins; cycloalkanes,naphthenes; and the heteroatomic compounds are lumped together as NSOs. Cycloalk-anes, aromatics, and NSOs can all have one or more alkyl side chains.11.1.1AlkanesParaffins come primarily from cracking of lipids.n-Alkanes of fewer than five carbonatoms are gases at ordinary temperatures and pressures. The largestn-alkane liquidat room temperature is heptadecane, C17H36(melting point 22±C). Octadecaneat.Downloaded from. Kaohsiung Medical University, on 17 Nov 2018 at 15:17:15, subject to the Cambridge Core terms of use, available
(melting point 28±C) and larger alkanes exist in solution in the petroleum liquid.

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